Implementing checkZdims to check no. random-effects versus no. observations

So this is my first pull request - hope I haven't done anything too stupid.

This version has adds a new function `checkZdims`parallel to `checkZrank` that is called immediately prior to this in `lFormula` and `glFormula`. The package builds, installs and checks apart from some tests that assume a Z-rank error message rather than the proposed error message.

Two things I haven't dealt with:
* warn, stop, ignore behaviour.
* unit tests 

Hope you find this useful.
/Rune